1. Field of the Invention
The invention relates to a forming die assembly for manufacturing a forged product.
2. Description of Related Art
A rare-earth magnet including a rare-earth element such as lanthanoid is also referred to as a permanent magnet. The rare-earth magnet is used in a driving motor and the like for a hybrid vehicle, an electric vehicle, and the like, in addition to a motor of a hard disc or a magnetic resonance imaging (MRI) device.
Examples of indices indicating magnetic performance of this rare-earth magnet include residual magnetization (residual magnetic flux density) and coercivity. Meanwhile, due to an increase in a calorific value caused by downsizing of a motor and high current density, demands for heat resistance of the rare-earth magnet in use have further been increased. Accordingly, retention of a magnetic property of the magnet used at high temperature is one of the major research themes in the technical field.
One example of a method for manufacturing the rare-earth magnet will briefly be described. Fine powder obtained from rapid solidification of Nd—Fe—B-based molten metal, for example, is subjected to hot pressing, and a sintered body is thereby produced. Then, in order to impart magnetic anisotropy to this sintered body, hot working is performed thereon. The rare-earth magnet (an oriented magnet) is manufactured in this way, and this method is generally used.
This hot working is a kind of hot forging, and the hot forging process includes extrusion, such as forward extrusion and backward extrusion, in addition to upsetting.
In particular, upsetting is generally used, in which a sintered body is disposed in a forming die assembly including at least an upper die and a lower die, the body is heated and pressed, for example, for approximately one second or less by the upper die, and the body is further pressed until a specified working ratio is obtained. A method for manufacturing a rare-earth magnet by imparting magnetic anisotropy to a sintered body through upsetting is disclosed in Japanese Patent Application Publication No. 2014-103386 (JP 2014-103386 A) and Japanese Patent Application Publication No. 2012-138507 (JP 2012-138507 A).
As for a conventional forming die assembly used for upsetting, the upper die and the lower die are integrally formed of steel or Inconel (registered trademark). Accordingly, when either the upper die or the lower die is damaged after repetitive use, the entire upper die and the entire lower die need to be replaced.
In addition, as for a forming die assembly used for closed-die forging, in which a workpiece is forged in the tightly closed forming die assembly, among upsetting, the upper die or the lower die is subjected to drilling and thereby provided with a recess for accommodating the workpiece. Accordingly, it is time-consuming to produce the forming die assembly, and as a result, production cost is increased.